Very cool. Though I felt that some sections might have been a bit too calm for a bossbattle (the part with the piano). But .. it creates a nice contrast between the other parts.
In terms of musicalness (?) and sound-design, you get a 5/5 imo.
Mixing wise I'd say that it sounds a bit too uneasy/restless/tiring, maybe because of the heavy compression. The track is a bit too long for heavy compression and I think you should give the listener a bit of a rest after a minute or so or maybe just lower the overall compression.
But, certainly not a bad track. I enjoyed listening to it!
-Avizura